This invention relates to protected metal articles and a method of producing a corrosion resistant composite coating. More particularly, the invention relates to a metallic coated ferrous strip of the type produced on a conventional hot dip coating line having polymeric fibers embedded in the metallic coating.
It is well known to provide corrosion protection to metal articles, such as steel sheets with a hot dip metallic coating. It is also well known the corrosion protection can be enhanced by dipping the metallic coated articles into a bituminous material. However, a bituminous coating does not readily bond to hot dip metallic coatings such as zinc, aluminum or lead. In the past, a thin asbestos felt or paper like layer of short asbestos fibers was embedded into a molten hot dip metallic coating. Random asbestos fibers became embedded in the solidified metallic coating and projected therefrom to provide an anchor for a bituminous coating. Hence, the primary purpose for bonding asbestos felt or paper to a hot dip metallic coated steel sheet is to form a composite metal-bituminous coating.
Such asbestos felt or paper for bonding a bituminous coating has been advantageously used over the years because it was inexpensive. However, there are several disadvantages. A first disadvantage is that the felt or paper is of very low tensile strength and easily tears when used in a continuous process. Accordingly, asbestos felt or paper is normally used as cut sheets in a more expensive batch type process. Steel sheets may be fed into a flux covered coating pot containing a bath of molten metal, such as aluminum or zinc. After a metallic coated sheet exits from the coating pot, a thin sheet of asbestos felt or paper is pressed into the molten coating metal using bonding rollers. The metallic coating is alloyed to the steel sheet and anchors the asbestos fibers. Hot dip metallic coating of short lengths of steel sheets using the flux process is very slow and produced at line speeds under 15 ft./min. (4.5 m/min.). Coating metal is wasted when excess molten metal is carried from the coating pot.
A second disadvantage is deterioration due to abrasion of the bonded asbestos felt or paper layer. When the asbestos layer is pressed into a molten metallic coating, the organic binders holding the fibers together are burned. Consequently, the embedded fiber layer has poor mechanical properties. Fabrication of coated steel sheets into articles tends to cause the fiber layer to craze or flake. Also, asbestos has very low tensile strength and individual fibers tend to break. Because of these physical characteristics, a coated steel sheet to be fabricated must have its asbestos layer impregnated with a saturant such as asphalt. The saturant provides protection to the fragile asbestos fibers during handling, shipping and fabrication of metallic coated steel sheets.
A third disadvantage of using asbestos is the environmental considerations. The potential health hazards related to breathing air laden with asbestos particles are well known. The asbestos fibers in the paper are bonded together using organic materials such as animal hair, wood pulp and resin. The galvanizing pot described above will be operated above a temperature of 830.degree. F. (443.degree. C.). The organic materials present in the asbestos felt readily burn giving off large amounts of smoke and noxious fumes. The saturating step described above also gives off smoke and noxious fumes as well.
A fourth disadvantage is the limited formability of hot dip metallic coated steel sheets having an asbestos felt or paper embedded in the metallic coating. It is well known to fabricate such steel sheets into building panels and riveted pipe. Pipe made by riveting is labor intensive since individual sections must be assembled by hand.
Unlike riveted pipe which is made from short lengths, helical formed pipe normally is made from a continuous length of strip. It has been suggested in the prior art that continuous asbestos felt or paper may be bonded to hot dip metallic coated strip. U.S. Pat. No. 3,077,032 suggests continuous asbestos felt may be bonded to ferrous strip emerging from a coating pot containing molten coating metal as the strip passes in a vertical direction. However, no one has commercially produced a hot dip metallic coated ferrous strip having an outer continuous asbestos felt or paper layer which was fabricated into articles such as helically formed pipe. Strip is metallic coated at line speeds much greater than that for sheets. Accordingly, a certain amount of back tension would have to be maintained on the paper roll as asbestos paper was fed into a bonding roller. Because of low tensile strength, the asbestos paper would frequently tear. More importantly, the organic binders in the asbestos paper immediately burn upon contact with the hot bonding roller and metallic coating. The paper tends to separate upon contact with either of these hot surfaces if the tension is too great or there is any interruption in the line speed of the as-coated strip. Even if such a continuous process were possible, the environmental problems in a workplace associated with asbestos paper are no longer acceptable.
The prior art as also suggested substituting other papers containing inorganic heat resistant fibers such as glass or carbon for asbestos. Unlike asbestos fibers, silica or carbon fibers do not bond well with metallic coatings, i.e. zinc. Even if they would bond, carbon or glass fibers are too brittle to be fabricated. Accordingly, paper made from fibrous carbon or glass is not an acceptable substitute for asbestos.
There has been a need for many years to find a heat resistant and strong fibrous felt or paper capable of being bonded with a hot dip metallic coated ferrous strip. There has also been a need for a paper which is environmentally acceptable in the workplace. We have discovered that a nonwoven organic fibrous paper made from heat resistant synthetic polymer fibers provides an improved bond with metallic coatings and its strength and toughness are maintained or even increased after contact with the molten coating metal. The improved toughness of the paper after bonding with the metallic coating allows the ferrous strip to be fabricated into building and construction products such as helically formed drainage culvert.